(1) Field of the Invention
The present invention relates to a battery voltage adjusting device and particularly, to a battery voltage adjusting device, which equalizes an on-vehicle high voltage battery consisting of a plurality of unit cells connected in series.
(2) Description of the Related Art
Recently, a hybrid electric vehicle (hereinafter, HEV), which travels using both of an engine and an electrically-powered motor, has been widely spread. The HEV is equipped with two kinds of battery, consisting of a low voltage (about 12 V) battery for starting the engine and a high voltage battery for driving the electrically-powered motor. The high voltage battery consists of a plurality of unit cells connected in series, wherein the unit cell includes at least one secondary battery such as a nickel-hydrogen battery or a lithium battery.
As for the high voltage battery, a state of charge (hereinafter, SOC) of each unit cell is equalized after an ignition switch is turned off, so that a dispersion of a SOC of each unit cell is reduced, thereby equalizing a performance of each unit cell. As a method of equalizing a SOC of each unit cell, for example, proposed are a method of adjusting a voltage by discharging a unit cell having a high voltage or a method in which a value of SOC, for making a dispersion of SOC of respective unit cells constituting a combined battery determined on the basis of a temperature of the combined battery be smaller than a predetermined dispersion, is set to be a target SOC value, thereby performing control, like a charge-discharge control device of a combined battery disclosed in Japanese Patent Application No. 2000-209782.
The equalization of a SOC is carried out by adjusting a dispersion of a voltage via detection of a voltage of each unit cell followed by a charge and discharge. However, since a unit cell (i.e. battery) experiences a voltage drop during a discharge due to an internal resistance as shown in FIG. 4, therefore a terminal voltage of the battery becomes lower than an open circuit voltage (hereinafter, OCV) that is a stable voltage. When the discharge is stopped, an electrochemical component of an internal resistance component remains as a voltage and the terminal voltage recovers with a time lag. That is, the terminal voltage of the battery does not immediately become an OCV after the discharge is stopped, but gradually approaches the OCV with a time lag. This time lag corresponds to a waiting time for obtaining a stable voltage (i.e. for obtaining stability of voltage), which is different depending on a temperature of the battery. During a charge, a similar phenomenon takes place as well. Therefore, when judging a dispersion of a voltage upon the equalization, the waiting time for obtaining a stable voltage must be taken into consideration since the equalization can be correctly carried out on a condition that the voltage is stable.
As described above, the waiting time for obtaining a stable voltage is different depending on a temperature of the battery. The lower the temperature of the battery, the longer the waiting time. Therefore, so far, the waiting time for obtaining a stable voltage is uniformly set taking the lowest temperature within a usable temperature range into consideration. However, in such a case, it also takes a long period of time before performing the equalization when the battery temperature is high, causing a problem that a dark current is increased.
In the charge-discharge control device of a combined battery disclosed in Japanese Patent Application No. 2000-209782, the target SOC for making a dispersion of a battery voltage converge is changed on the basis of the battery temperature and no measure is taken into consideration as to the waiting time for obtaining a stable voltage. Therefore, there is a problem that it takes a long period of time before performing the equalization when the battery temperature is high or a problem that the voltage is detected when the voltage is not stable, causing an increase in a dark current and a problem that a correct equalization cannot be carried out.